Severe restrictions are placed on the alloys which are used for restorative dentistry because of the particular requirements of such alloys with respect to various properties thereof which are essential for the particular use to which the alloys are put. In general, the alloys which have conventionally been found to be most suitable are gold alloys or platinum-containing gold alloys of high quality. These alloys have been most used not only because of the traditional preference of the gold color but also because the gold has extremely high chemical stability against discoloration and dissolution in the mouth as well as having various preferred mechanical properties such as high extensibility, high workability and high wear resistance. However, while gold has been found to be most suitable, the price is extremely high and various substitutes have been sought.
Among the alloy substitutes for gold, the alloys containing copper as a principal ingredient and having a gold color have been considered and researched with respect to the desire of obtaining those properties of the alloys which are as similar as possible to the properties of the gold alloys.
These substitute alloys which are used for restorative dentistry are generally used in forms such as inlay, a crown, a clasp and a bridge, and the clinical value thereof depends, for example, in the case of the inlay, on the extent to which the alloy can prevent the tooth to which it becomes associated from again becoming decayed, as well as how the alloy can maintain the shape and function of the tooth. Most important, is the length of time during which the alloy can be effective for these purposes.
From the clinical standpoint, the physical properties which can protect the alloys against breakage and deformation are such wherein the tensile strength is greater than 40kg/mm.sup.2, an extensibility greater than 10% and a hardness greater than 180 (Vickers hardness units). These are considered the optimum characteristics, however, the high degree of hardness is actually too high and is rather inconvenient in practice.
Furthermore, a conflicting relationship exists between the tensile strength and the extensibility, and the difficulty in manufacturing of the alloys for restorative dentistry is to a great extent caused by the need to balance each of these properties with the other. The advantage of the high quality gold alloys, as previously mentioned, exists in this respect, i.e. in the fact that the tensile strength and extensibility which are actually in conflict with each other are mutually balanced in the case of the high quality gold alloys while presenting sufficiently high values to provide satisfactory properties.
There have been considerable difficulties in providing substitute alloys which meet this requirement. Although the gold-silver-palladium alloys exhibit properties similar to those of the gold alloys, the problem with these gold-silver-palladium alloys is that the same also exhibit melting points which are unsuitably high for casting. In addition, these alloys are also relatively high in price.
On the other hand, various copper alloys containing copper as the principal ingredient and also exhibiting a gold color have been prooposed and considerable research has been conducted to obtain properties thereof which are as similar as possible to the optimum properties.
One of these gold color copper alloys, referred to as "Neoden" exhibits many of the preferred properties. Neoden consists, by weight, of 51% of copper, 47% of zinc, 1.0% of indium, 0.2% of tellurium and 0.8% of silicon, melted together into an alloy which is claimed to be as good as 14K gold alloy. Also, compared to the 14K gold alloy with silver, which has principally been used only for relatively simple inlays because of the hardness and elasticity thereof, Neoden is also claimed to be useful as an inlay for relatively complicated decayed cavities and also for crowns and bridges of high adaptability. In addition to this wide range of application, Neoden is also of course claimed to be advantageous from the standpoint of economy since the material price thereof is approximately 1/6 the price of 14K gold alloy approximately 1/6 the price of gold-palladium alloy.
Another gold color copper alloy which has been developed particularly because of the color tone thereof consists by weight of 47 to 85% of copper, 10 to 50% of zinc plus 0.3-15% of indium, 0.02 to 3% of tellurium and 0.2 to 3% of silicon. This alloy has also been used for restorative dentistry and is as good as Neoden with respect to extensibility, adaptability for casting and workability. This gold color copper alloy has a high elasticity and has accordingly found to be suitable especially for use as a cast spring and as a one piece cast for restorative dentistry. However, it has not been found suitable for other applications.
Until the present, Neoden has been considered to be the most preferred gold color copper alloy in view of the versatility thereof, the tensile strength and the extensibility. However, Neoden has encountered a very serious problem, particularly with respect to the tendency thereof to discoloration, a low corrosion resistance and consequently a tendency to breakage or damage due to corrosion. Also, with respect to hardness, Neoden is slightly too hard to be easily worked and is inferior to the gold alloys. Furthermore, during casting the formation of air bubbles often occurs and this is also a problem in the manufacturing of the alloys. Of the various disadvantages, the tendency to discoloration is the most serious from the aesthetic viewpoint and considerably lowers the value of the alloy from the commercial standpoint. Experiments have shown that although it might be possible to obtain products of satisfactory quality free from discoloration when producing the alloy on a laboratory scale from materials totalling approximately 200g in weight, the products obtained in mass scale from the materials of 1kg or more exhibit a tendency to darkening once the product has been utilized as an inlay, for example, in a decayed cavity of the tooth. This fact has posed a problem for the maintenance of quality in mass production and severely restricted the use of Neoden commercially.